Composite coatings

ABSTRACT

The invention describes composite coatings, in particular comprising carbon and another metallic element such as silicon or aluminium. These coatings have improved properties compared with pure tetrahedral amorphous carbon coatings, in that they have reduced stress levels and can be deposited at higher thicknesses, whilst retaining acceptable hardness and other useful mechanical properties. Also described are methods of making composite coatings, materials for making the coatings and substrates coated therewith. Specifically, a method of applying a coating to a substrate using a cathode arc source, comprises generating an arc between a cathode target and an anode of the source and depositing positive target ions on the substrate to form the coating, wherein the coating is a composite of at least first and second elements and the target comprises said at least first and second elements. A composite coating comprises tetrahedral amorphous carbon and a metallic element other than carbon, the composite coating having an sp 3  content of at least 60%. A target for use in a cathode arc source comprises a mixture of carbon and a metallic element other than carbon.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.09/336,753, filed Jun. 21, 1999 (allowed) know U.S. Pat. No. 6,143,142.

The present invention relates to composite coatings, in particular suchcoatings comprising carbon and another metallic element. The presentinvention relates also to methods of making composite coatings,materials for making the coatings and substrates coated with thecoatings.

Amorphous silicon-carbon alloys (a−Si_(1.x)C_(x)) have attracted muchrecent attention not only due to the composition dependent variabilityof their optical band gap but also because of their important role asintermediate layers for the growth of diamond films on crystallinesilicon and non-diamond substrate. Several attempts have been made todeposit (a−Si_(1.x)C_(x)) films using existing thermal chemical vapourdeposition (CVD) or plasma assisted CVD techniques, but these techniquesinvolve high deposition temperatures which may destroy or damage manysubstrate materials. Also, known CVD techniques have used metal organiccompounds, undesirable due to their toxicity.

It is known to deposit hard thin films, such as tetrahedral amorphouscarbon (ta-C), using a filtered cathode arc (McKenzie et al 1991, Fallonet al 1993, Martin et al 1988). These ta-C films have interesting anduseful properties, such as extreme hardness (˜70 Gpa), thermalstability, high electrical resistivity, wide Tauc optical band gap(˜2.5eV), smooth surface and low friction, and transparency in widespectral range because of the high sp³ fraction of carbon atoms (up to87%) in the film.

However, the high internal stress in the films can limit theirapplications, especially when it is desired to deposit a relativelythick film, as the film may flake away from the substrate.

In order to reduce the internal stress of ta-C films, and in an attemptto improve adhesion of thick films of this type, different modificationshave been made, such as nitrogen incorporation into the films. However,whilst the internal stress can be reduced a little, this is notsufficient to enable significant increases in usable film thickness. Inaddition, there are disadvantages to incorporation of nitrogen intothese films as so doing can harm many of the mechanical properties ofthe films.

Metal-containing diamond-like-carbon (DLC) materials are knownpotentially to have useful electrical and mechanical properties, wearresistance and friction (Dimigen et al 1987). It has been reported thatsuch films containing certain low percentages of metals can havecomparable wear resistance and friction coefficient with the a-C:Hfilms, and may have better adhesion to the substrate. Introducingcertain metal elements such as aluminum into the DLC films may reducefilm stress, but only at the unacceptable expense of its mechanicalproperties, such as hardness and Young's modulus.

It is therefore an object of the invention to provide composite coatingsthat solve or at least ameliorate the aforementioned problems. Inparticular it is an object of specific embodiments of the invention toprovide composite coatings that exhibit reduced stress, thus enablingdeposition of relatively thick coatings whilst retaining acceptablehardness.

Accordingly, the present invention provides, in a first aspect, a methodof applying a coating to a substrate using a cathode arc source,comprising:

generating an arc between a cathode target and an anode of the source;and

depositing positive target ions on the substrate to form the coating,

wherein the coating is a composite of at least first and second elementsand the target comprises said at least first and second elements.

Thus, the invention enables the production of composite coatings fromtargets used in a cathode arc that contain two or more coatingcomponents. It is an advantage of the method that composite films caneasily be produced using the filtered cathode arc process, and withoutthe need for introduction of gaseous compounds into the arc vacuumchamber. Composite films were previously made using, for example, agraphite target and hydrocarbon gas, SiH₄ gas or a metal organiccompound in vapour form introduced typically close to the substrate. Theresultant films had high hydrogen content and suffered from poormechanical properties. The method of the invention avoids the necessityfor gaseous components and enables production of films that have lowerhydrogen contents than and improved mechanical properties than possiblehitherto. Films of the invention typically have a hydrogen content of20% or less, preferably 10% or less, and in specific embodiments of theinvention substantially hydrogen-free coatings are produced.

It is a further option for the method to deposit a coating that is acomposite of at least first, secondhand third elements and wherein thetarget comprises said at least first, second and third elements.Alternatively, the coating can be a composite of at least first, secondand third elements and the target comprises said at least first andsecond elements and the method comprises introducing the third elementinto the coating in a gaseous or liquid form.

It is envisaged that the method of the invention is of applicationwithout limit to the choice of target materials. Specifically, themethod has successfully been carried out using a target that comprisescarbon, producing a coating of a composite comprising tetrahedralamorphous carbon. The target preferably contains, as second element, ametal other than carbon. The target should be electrically conducting,so other target materials may be chosen that are non-metallic, providedthat the target is sufficiently conducting to be used as a cathodetarget in a cathode arc deposition apparatus. Where the second elementis a metal it is suitable selected from titanium, nickel, chrome,aluminum, silicon and tungsten. Reference to element is intended to bereference to the element whether present in elemental or ionic orcompound form.

In a particularly preferred embodiment of the invention the methodcomprises depositing a layer of a composite film of carbon and silicon,suitably using a target which contains at least 40% carbon, theremainder being substantially silicon. The composite Si—C film obtainedhas uses in the semiconductor field. Also the Si—C film obtained can beused for its improved trabelogical properties of reduced stress and highhardness compared to known DLC and DC-based films.

In a further particularly preferred embodiment of the invention themethod comprises depositing a layer of a composite film of carbon andaluminium, suitably using a target which contains at least 80% carbon,the remainder being substantially aluminum.

The use of composite targets has the advantage that it is possibleaccording to the invention to deposit coatings that have a highproportion of sp³ bonds. It is preferred that the deposited coating hasan sp³ content of at least 60%, more preferably at least 70%, and inspecific embodiments of the invention sp³ percentages of 80% and aboveare achievable.

The invention additionally provides in the first aspect a method ofdepositing a composite coating of at least first and second elements,comprising:

generating an arc between an anode and a cathode target, wherein thecathode target comprises said first and second elements, so as togenerate positive ions of said first and second elements; and

depositing said ions on a substrate to form the composite coating.

The target used in the method can comprise carbon and the compositecoating comprise tetrahedral amorphous carbon having an sp³ content ofat least 70%.

In a second aspect of the invention there is provided a compositecoating comprising tetrahedral amorphous carbon and a metallic elementother than carbon, the composite coating having an sp³ content of atleast 60%. The sp³ content in preferred coatings is at least 70%.

In embodiments of the invention a composite coating comprises 99.9-80%carbon and 0.1-20% aluminium. These have been found to exhibitparticularly desirable properties as more specifically set out in theexamples below.

In further embodiments of the invention a composite coating comprises99.9-40% carbon and 0.1-60% silicon. These have been found to exhibitparticularly desirable properties as more specifically set out in theexamples below.

An advantage of films of the invention is that they have stress levelsthat are reduced compared to pure ta-C films, and therefore films of theinvention can be deposited at greater thicknesses than pure ta-C films,but retain an acceptable hardness. In terms of their structure, films ofthe invention retain a significant proportion of the structure seen inpure ta-C films, such as a high level of sp³ bonding. The films ofspecific embodiments of the invention have additionally been found toexhibit increased adhesion to substrates and to have a good coefficientof friction. The coatings are of use in applications where suchproperties are sought, in particular on forming tools, lift frames forsemiconductor chips, components of moulds especially injection moulds,dies and punches.

In a third aspect of the invention there is provided a substrate coatedwith a composite coating according to the second aspect of theinvention. It is an advantage of the invention that hard thick coatingsare obtainable, and the composite coating typically has a thickness ofup to 10 microns.

A still further, fourth aspect of the invention provides a target foruse in a cathode arc source, comprising a mixture of carbon and ametallic element other than carbon.

The target may comprises carbon and silicon, and may comprise carbon andaluminum. Targets of this composition have been used in specificembodiments of the invention as described in more detail below,resulting in films have advantageous properties. The target may alsocomprise carbon and another element, such as a metallic element selectedfrom titanium, chromium, nickel and tungsten.

To obtain a target of the invention, a mixture of carbon and themetallic or other element can be sintered, for example in the presenceof a binder such as bitumen or tar.

The invention yet further, in a fifth aspect, provides a method ofmaking a target for use in a cathode arc source, comprising:

combining at least first and second target components in powdered and/orfinally divided form to produce a mixture of said first and secondtarget components; and

pressing said component to form a target.

The method preferably comprises sintering the mixture of first andsecond target components at elevated temperature, more preferably at atemperature of 1000° C. or higher.

It is optional to include a binder, in which case the method comprisescombining the target components in the presence of the binder, such astar, bitumen, alcohol and mixtures and compositions thereof.

Clean amorphous silicon-carbon (a-SiC) alloy films were thus depositedin specific embodiments of the invention by filtered cathodic vacuum arctechnique. The silicon content in the film was determined by X-rayphotoelectron spectroscopy (XPS) measurement and found int examples tovary from 2.4 to 48 at. %. Both XPS and Raman measurements showed theexistence of amorphous silicon carbide clusters in the film with siliconcontent between 42 and 48 at. %. With increasing silicon content, thehardness of the film decreases from 62 Gpa to 22 Gpa while thecompressive stress decreases from 8.2 Gpa to 2.0 Gpa.

Aluminium-containing tetrahedral amorphous carbon (ta-C:Al) films weresimilarly thus prepared according to the invention in a filteredcathodic vacuum arc process. Characterization of the films was mainlyfocused on their mechanical properties and internal stress in terms offilm structure and Al content. The film structure was studied mainly byMicroRaman Spectroscopy. The mechanical properties were measured bynanoindentation testing. The internal stress was evaluated with a radiusof curvature technique by means of surface profilometry. It was noticedthat the internal stress of example films was reduced significantly from10-12 GPa in the ta-C films to 1-2 GPa in the ta-C:Al films. However,the hardness of films had a drop to around 25 GPa when the Al content inthe films was beyond 10 at. %. From the Raman measurement, the ratios ofD-peak intensity, full width at half height (FWHH) and peak area tothose of G-peak slightly increased with the increase of Al content. Thisindicates that the amount of sp2 bonding in these films has increased.It appears that the increase of sp2 bonding is not considerable.However, with the further increase of Al in the films, the D-peakdeveloped much faster than the G-peak, meaning that the sp2 componenthas significantly increased and indicating that in films of theinvention Al can effectively be doped in the film as an acceptor, andthen the sp³ bonding is maintained, and that Al alloying such as Al₄C₃or AIOC can happen when excess Al is introduced into the film. Then Alcan exist in clusters in the sp³-dominant carbon network.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now illustrated in specific embodiments with referenceto the accompanying drawings in which:

FIG. 1 shows deconvoluted C 1s XPS spectra for amorphous silicon-carbonalloy films with different silicon contents;

FIG. 2 shows Raman spectra of amorphous silicon-carbon alloy films withdifferent silicon contents;

FIG. 3 shows the results of meaurement of properties of depositedcarbon-silicon composite films, namely:

3 a shows film stress and hardness as a function of silicon content inthe film;

3 b shows Raman shift as a function of silicon content in the film;

3 c shows variation in silicon content in the film against siliconcontent in the target;

3 d shows stress against hardness in silicon-carbon composite films;

3 e shows hardness and Young's modulus as a function of silicon contentin the films;

FIG. 4 shows a surface morphology of films: (a) ta-C film (prior art);and (b) ta-C:Al film deposited using a C/Al target containing 5 at. %Al;

FIG. 5 shows Raman shift against aluminum content of carbon-aluminumcomposite films;

FIGS. 6-8 show the results of measurement properties of depositedcarbon-aluminum composite films, namely:

FIG. 6 shows hardness and Young's modules as a function of aluminumcontent in the films. The data illustrated in FIG. 6 are derived fromseparate data illustrated in FIGS. 7 and 8. FIG. 6 shows the variationof hardness and the Young's modules verses compressive stress forta-C:Al at different Al contents. It indicates that for low compressivestress coatings, higher Al compositions are needed ranging from 5.0 at.% Al to 15.0 at. % Al. With these higher Al compositions, coatings withrelatively high hardness and low compressive stress are obtained;

FIG. 7 shows hardness as a function of aluminum content at a substratebias of −80V; and

FIG. 8 shows stress against hardness in aluminum-carbon composite films.

EXAMPLE 1

The amorphous silicon-carbon alloy films were deposited by a FCVA systemdescribed elsewhere. Carbon and silicon ions are produced in a vacuumarc discharge between the cathode and the grounded anode. The cathode isa 60 mm diameter target mounted on a water-cooled stainless-steel block.Pure graphite and silicon powder (325 mesh) with different atomicfractions were mixed thoroughly and compressed to cylinder shapedtargets by a pressure of 770 MPa. The arc current was kept constant at90 A. A toroidal magnetic field around 40mT was employed to produce theaxial and curvilinear fields to steer the plasma. All depositions werecarried in floating condition at room temperature and with vacuumpressure less than 10⁻⁶Torr. The substrate was clean (100) n-typesilicon with average thickness of 0.5 mm. XPS measurement was carriedout on a VG Scientific Microlab 310F system using a Mg Ka (1253.6 eV) asthe x-ray source. The Raman spectra were excited using the 514.5 nm lineof an Ar⁺laser and collected with back scattering on a CCD camera usinga Renishaw micro-Raman System 1000 spectrometer. The film stress wasdetermined by a surface profilometer (Tencor P10) with theradius-of-curvature method. The hardness of the film was measured by anindenter (Nano-lndenter®II) operated in a constant-displacement-ratecontinuous stiffness mode.

All the silicon-carbon alloy films exhibit a clean and smooth morphologywith RMS roughness smaller than 0.6 nm over an image area of 1, μm². Thesilicon contents (Si/(Si+C)) in the films determined by XPS measurementare 2.4, 14.5, 29, 42 and 48 at. %, while the silicon contents in thetargets are 1, 5, 10, 20 and 30 at. %, respectively. The larger siliconcontent in the film compared with in the corresponding target may beresulted from the lower melting point of silicon compared with graphite.FIG. 1 shows the XPS narrow scan of C 1s peak for amorphoussilicon-carbon alloy films. The C 1s peak was deconvoluted into threecomponents located at 283.2, 284.5 (285.0) and 286.3 eV, which areattributed to C—Si, C—C and C—O (contamination) bonds, respectively. TheC—C bond is located at 284.5 eV for the films with 14.5 and 29 at. %silicon, but at 285.0 eV for the films with 42 and 48 at. % silicon. Itwas observed that the C is peak for ta-C films contains a sp³ C—C bondat 285.4 eV and a sp² C—C bond at 284.3 eV. The higher C—C bond positionfor the present films with 42 and 48 at. % silicon indicates a higherfraction of sp³ bonded carbon. As the silicon content increases, thepeak area of the C—Si bond increases. In the films with 14.5 at. %silicon, the C—C bond is the main bond for carbon atom. In the filmswith 42 and 48 at. % silicon, the C—Si bond becomes the dominant bond.The Si 2p peak (not shown) was deconvoluted into a big component at100.5 eV and a small component at 102.7 eV. The former is attributed toSi—C bond, and the later is to Si—O bond. The Si—C bond is predominantin the Si 2p peak for all films with different silicon contents.

The Raman spectra for the films with different silicon contents areshown in FIG. 2 in the range of 600 to 1800 cm⁻¹. The spectra have beendisplaced vertically for clarity. For the films with silicon contentfrom 2.4 to 29 at. %, a broad band predominates in the range of1400-1700 cm⁻¹, and a wide peak appears around 950 cm⁻¹. The former peakis due to the vibrational mode of amorphous carbon clusters (Schwan etal 1996) and the later is the second order Raman vibrational modes ofsilicon substrate. For films with silicon content of 42 and 48 at. %, abroad band appears in the range of 600-900 cm⁻¹, which is considered asthe vibrational mode of the amorphous silicon carbide cluster (Zhang etal 1998, Kumbhar et al 1995). The broad peak in the range of 1400-1700cm⁻¹ was fitted with two Gaussian peaks defined as the graphite (“G”)and disorder (“D”) peaks, respectively. The fitting shows a big G peakand a small D peak. The G position almost linearly decreases from 1571cm⁻¹ for the film containing 2.4 at. % silicon to 1416 cm⁻¹ for the filmcontaining 42 at. % silicon. It could be a good indicator for the filmcomposition. For the films containing 48 at. % silicon, the Raman bandof the carbon cluster appears around 1370 cm⁻¹ and becomes very weak.The great decrease of the G position of the carbon cluster isqualitatively explained as following. At low silicon concentration, thesilicon atom predominately substitutes the carbon atom. As the atomicfraction of silicon in the film increases, more and more silicon atomssubstitute the carbon atoms into the ring shaped sp²-bonded carboncluster. As the silicon atom is heavier than the carbon atom and Si—Cbond is weaker than C—C bond, the vibration energy of stretching mode ofthe ring becomes lower and lower.

For the film containing 42 at. % silicon, a strong broad peak due to theamorphous silicon carbide cluster appears around 790 cm⁻¹. For the filmcontaining 48 at. % silicon, this peak is centred at 750 cm⁻¹. Theappearance of the wide peak around 750-790 cm⁻¹ suggests that there areseparated silicon carbide clusters in these films. IR absorptionmeasurement of amorphous silicon-carbon films deposited by hot-filamentassisted CVD method (Kumbhar et al 1995) showed that the absorption peakcorresponding to Si—C vibration shifts from 760 to 800 cm⁻¹ as thesilicon content decreases. This is in good agreement with our result.The higher peak position for the film with 42 at. % silicon may beresulted from that the amorphous silicon carbon clusters are stillcarbon rich.

With increasing silicon content, the internal compressive stress of thefilm decreases monotonously from 8.2 Gpa for the film with 2.4 at. %silicon to 2.0 Gpa for the film with 48 at. % silicon (FIG. 3). Thehighest stress 8.2 Gpa is very near the value of pure ta-C film. Astress of below 2.5 Gpa in the film with 42 and 48 at. % silicon enablesto deposit thick films with a thickness over 500 nm. The hardness showsalmost similar behaviour as the internal stress, decreasing from 62 GPafor the film with 2.4 at. % silicon to 22 Gpa for the film with 48 at. %silicon. Quite high hardness for the films with 42 and 48 at. % silicon(24 and 22 Gpa) may be due to the existence of amorphous silicon carbidecluster and the subplantation deposition mechanism of FCVA technique.

Hydrogen-free clean and hard amorphous silicon-carbon alloy films havebeen successfully deposited according to the invention. Both XPS andRaman spectroscopy show the existence of silicon carbide cluster inspecific films with silicon content between 42 and 48 at. %. The siliconatoms predominately substitute the carbon atoms into the carbon clusterat low silicon concentration, and form amorphous silicon carbide clusterat a higher silicon concentration.

EXAMPLE 2

The ta-C:Al films were deposited in a filtered cathodic vacuum arc(FCVA) process. The FCVA system has been addressed elsewhere(Shi et al1996). The mixed aluminum/graphite (Al/C) targets were used in place ofpure graphite target during deposition. The Al/C targets with thevarying Al content were made of Al and graphite powders under a pressureof 0.6 GPa. The ta-C:Al films were deposited at a bias of −80 V. In theFCVA process, the ionized atoms produced from the target in the vacuumchamber (10⁻⁴ ˜10⁻⁷ Torr) were accelerated through amechanical-electrical-magnetic filtering bend towards the substrate andfurther deposited on the substrate. Undoped, n-type <100> silicon waferswere used as the substrates. The wafers were first cleaned in detergentliquid and then in deionized water using an ultrasonic machine beforeentering the vacuum chamber. The substrate surface was further cleanedby Ar ion bombardment in the vacuum chamber prior to deposition. Thewafer surface was then coated a layer of ta-C:Al film. All thedepositions were carried out at the room temperature.

A micro-Raman spectroscope (Ramascope, Renishaw) with 514.5 nm Ar laserwas used to characterize the film structure. The laser output power was10 mW. A filter of 50% was also used. The laser beam was focused on thesample surface using an optical microscope with a magnification of50×(laser spot size˜1 μm). The Raman spectra were acquired in the rangeof 1100 cm⁻¹ and 1900 cm⁻¹ to evaluate the differentiation of structuresof ta-C:Al films in terms of the first-order D and G peak positions, andthe ratios of peak amplitudes, full width at half maximums (FWHM), andintegrated peak areas between the corresponding D and G peaks.

Atomic force microscopy (AFM) (S-3000, Digital Instruments) and surfaceprofilometry (Tencor P-10) were used to measure the surfacemorphological characteristics of films.

SEM/EDX and XPS were used to analyze the Al content in the ta-C:Alfilms.

The wear and scratch tests were also performed on the ta-C:Al filmsusing a pin-on-disc tribometer (CSEM) and a microscratch tester (CSEM),respectively.

The micro-Raman spectroscopy is a non-destructive method for measuringthe bonding structure of the materials. Since the 514.5 nm laser lightis more sensitive to the sp² carbon bonding, the sp² contribution isalways explicit in the Raman spectrum. Consequently, the Raman spectrumof the ta-C films is dominated by the G peak at about 1550 cm⁻¹ and theD peak at around 1350 cm⁻¹, both of which are attributed to the sp²bonding.

FIG. 4 shows the AFM images of surface morphology of ta-C and ta-C:Alfilms, where the ta-C films were deposited at a bias of −80 V and theta-C:Al films were grown using a C/Al mixed target containing 5at. % Al.It can be seen that the ta-C film contains fine asperities. The particlesize of ta-C:Al film comes larger. It is hypothesized that the fineparticles in the ta-C film are due to a large percentage of sp³ carbonbonding formed by the high impinging carbon energies. For the ta-C:Alfilms, the content of sp² bonding has appreciably developed due to thehigh Al content in the films, which leads to the relatively largeparticles in the films.

The Raman spectrum results from inelastic scattering of photons. Thetotal intensity of a Raman signal depends on Raman scattering crosssections, beam geometry, excitation power, detection efficiency.

Typically in the ta-C films, the predominant component is sp³ bondedcarbon together with certain sp² bonded carbon clusters. The Ramanphonon lines are more sensitive to the sp² carbon bonding due to itslarger Raman scattering cross-section. The Raman spectra for the filmsin this study give a broad band overlaid by G and D peaks as shown inFIG. 5.

For the Al containing films, both the G and the D positions shift to thelower frequencies with the increase of Al content. This trend may becaused by the Al induced stress release or by the excess Al content.Another study has shown that the internal stress has been reduced from10-12 GPa for the ta-C films to 1 to 2 GPa for the film deposited with aAl/C target containing 15 at. % Al. Ager III et al. have reported a Gpeak shift of about 20 cm⁻¹ on a ta-C film deposited at −100 V bias whencomparing the strained film with the free standing film. However, the Gpeak shift is up to about 90 cm⁻¹ for the Al containing ta-C films inthis study. It is therefore inferred that the G peak position shift isnot only induced by the internal stress release.

The ratio of the intensity of D peak to the intensity of G peak can beused to reflect the extent of graphitization of the DLC films, i.e. theratio I_(D)/I_(G) increases as the amount of the sp² bonded clusters inthe sp³ bonded matrix increases. (assuming that the intensity of the Gpeak remains relatively unchanged).

FIG. 5 shows the position of the D and G peaks as the aluminum contentis varied in the films. The graph shows that both peaks change inposition as the aluminum content in the film is increased. The D peakshifts from 1406 cm⁻¹ to 1327 cm⁻¹ when the Al content is increased from0.7 at. % to 15 at. %. The G peak shifts from 1569 cm⁻¹ to 1487 cm⁻¹.

For the peak intensity and area ratios, the measurements indicate thatboth are increasing as the content of aluminum increases. To explain thephenomenon, we propose to classify the clusters into two categories, oneranging from atoms to nanoclusters and the other from nanoclusters tomacroparticles.

The shift of G peak position to a lower frequency can be used toestimate the reduction of the compressive stress as aluminum isintroduced into the films. The hypothesis for this estimation is thatthe vibrational frequency in a material is proportional to itsinteratomic forces between the atoms. If the material is strained thespacings between the atoms change correspondingly, which further resultsin the change in the vibrational frequency.

The shift of the D peak position to a lower frequency may be due to thefurther increase of the degree of disorder of the sp² bonded clusters.The bond angles of the bonds in the clusters get also further distortedand the bond lengths increase, which weaken the bond strength. Thisgives rise to the lowering of the D peak position. The increase indisorder also means that the size of the clusters is reducing while theintensity of the D peak increases and the intensity of the G peakdecreases. This implies that the ratio of peak intensities of D and Gpeaks should increase as the amount of aluminum increases (FIG. 7a).

The increase in the D peak intensity may also be related to the relativeincrease in the amount of sp² bonded clusters embedded in the sp³ bondedmatrix in the ta-C:Al films,. The differentiation in the ratio of FWHMsis relatively small compared to the other two ratios.

The normal ta-C films are composed of the sp³ bonded carbon matrix withthe segregated sp2 bonded carbon clusters. By introducing aluminum, itis expected that more doubly bonded carbon clusters or even Al clustersmay be created to release the compressive internal stresses in the filmsin expense of amount of the tetrahedral bonds. The Al clusters are alsoexpected to absorb some strain in the film.

From the Raman measurement, the ratios of D peak intensity, FWHM andpeak area to those of G peak slightly increase with the increase of Alcontent. It means that the amount of sp² bonding in these films hasrelatively increased. With a small Al content, the increase of amount ofsp² bonding appears to be not appreciable. However with a relativelyhigh Al content in the film, the D peak develops faster than the G peak.It means that the sp² component in the ta-C:Al films containing more Alhas considerably increased relative to the sp³ component. Two hypothesesare therefore proposed. The first one is that the amount of tetrahedralbonding may be maintained less changed as the Al content is small. TheAl atoms may also partially act as the electron acceptors forming thedoped structures. The second is that the Al atoms in the film may reactwith C to form Al/C compounds or exist in the Al clusters when theexcess Al is introduced into the film, which is being investigated.

There was a tendency that the ratios of intensities, FWHMs and peakareas between D and G peaks first decreased and then increased with theincrease of substrate bias during deposition. The minimum ratios wereobtained at a bias around −80 V, which corresponds to the maximum sp³content in the ta-C films.

For the ta-C:N films, the ratios first slowly increased with theincrease of N₂ partial pressure when the N₂ partial pressure was lowerthan 1×10⁻⁴ Torr, and then increased relatively fast with the increaseof N₂ partial pressure when the N₂ partial pressure was higher than1×10⁻⁴ Torr. The amount of sp² component may become much higher when theN₂ partial pressure is higher than 1×10⁻⁴ Torr.

For the Al containing films, both the (G and the D peak positionsshifted to the lower frequencies with the increase of Al content. Thepeak intensity and area ratios increased as the content of aluminumincreased. The differentiation in the FWHM ratio was relatively smallcompared to the other two ratios. More doubly bonded carbon clusters oreven Al clusters may be created in expense of amount of the tetrahedralbonds. The Al clusters were also expected to absorb some strain in thefilm. It is proposed that the amount of tetrahedral bonding in the filmsmay be maintained less changed as the Al content was small. The Al atomsin the film may react with C to form Al/C compounds or exist in the Alclusters when the excess Al was introduced into the film.

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What is claimed is:
 1. A composite coating comprising tetrahedralamorphous carbon and an element other than carbon, the composite coatinghaving an sp³ content of at least 60%.
 2. A composite coating accordingto claim 1 having an sp³ content of at least 70%.
 3. A composite coatingaccording to claim 1 comprising 99.9-80 atomic % carbon and 0.1-20atomic % silicon.
 4. A composite coating according to claim 1 comprising99.9-40 atomic % carbon and 0.1-60 atomic % silicon.
 5. A substratecoated with a composite coating according to any of claim
 1. 6. Asubstrate according to claim 5 wherein the composite coating has athickness of up to 10 microns.